This invention relates in general to braking systems, and in particular to an algorithm for electronically controlling poppet valves in an electrohydraulic brake system to control the pressure of brake fluid applied to vehicle wheel brakes.
Traditional hydraulic brake systems include a brake pedal operated by the driver of a vehicle. The brake pedal operates a master cylinder, causing the master cylinder to send pressurized hydraulic brake fluid to the wheel brakes of a vehicle. This is sometimes referred to as foundation or base braking—the basic braking called for by the operator of a vehicle. Over the years, engineers have worked to improve the performance of the braking system of vehicles by augmenting or replacing the base braking function with another braking operation.
Electrohydraulic brake (EHB) systems utilize electronically controlled valves, pumps, or a combination thereof to augment, replace, or control the base braking operation of a vehicle brake system. One of the first of many advanced braking functions that has been developed for vehicles is ABS (Antilock Braking System), which typically involves the operation of valves and pumps to selectively release and re-apply brakes during a braking operation. While typical base braking is commanded by the operator, ABS braking controls the vehicle brakes to recover from and limit skidding of a vehicle's wheels due to braking the wheels harder than permitted by the available coefficient of friction of the road surface. Since pumps and valves are electronically controlled to augment the base braking operation, a vehicle equipped with ABS may generally be said to have an EHB system. Another advanced braking function that may be accomplished by a properly configured EHB system is VSC (Vehicle Stability Control), which is a system for selectively actuating vehicle brakes to improve the stability of a vehicle during vehicle maneuvers. Other braking applications producing a pressure command input to the present invention include DRP (Dynamic Rear Proportioning—a system for controlling the front to rear proportioning of a vehicle braking command), TC (Traction Control—which typically involves selective application of brakes during vehicle acceleration to recover from and limit skidding of a vehicle's wheels due to accelerating the wheels faster than permitted by the available coefficient of friction of the road surface), ACC (Autonomous Cruise Control—a cruise control system that can actuate vehicle brakes to maintain proper vehicle spacing relative to a vehicle in front) and similar functions.
Various forms of electrohydraulic braking systems have been proposed. For example U.S. Pat. No. 5,941,608, Campau, et al. discloses an electronic brake management system with manual fail-safe capabilities. A subset of electrohydraulic braking systems is an electronic brake management system (EBM). EHB systems can allow braking to be primarily controlled by the vehicle driver with a conventional master cylinder system. Additionally, an electronically controlled portion of the system operates the brakes under certain conditions, i.e. anti-lock, traction control, etc. Primary braking is controlled electronically in Electronic Brake Management systems. Normally, the vehicle driver or a safety system generates an electronic signal, which in turn operates the pumps and valves to achieve a braking pressure within the system. A pedal simulator creates the effect for the driver of applying direct braking pressure while also providing a back-up braking system in case of a failure of the primary system. In the back-up system, the pedal simulator acts as a master cylinder during the failure event and provides the hydraulic pressure that actuates the brakes.
In Campau, et al., as well as in many EHB systems, there are a series of valves in the hydraulic circuit that operate to apply pressure to or dump pressure from the brake. In one embodiment of Campau, proportional control valves are provided for each vehicle brake. In a first energized position, an apply position, a proportional control valve directs pressurized hydraulic fluid supplied to the proportional control valve from a fluid conduit to an associated fluid separator unit. In a second energized position, the maintain position, the proportional control valve closes off the port thereof which is in communication with the associated fluid separator unit, thereby hydraulically locking the associated fluid separator piston of the fluid separator unit in a selected position. In a de-energized position, the release position, the spool of the proportional control valve is moved by a spring to a position where the proportional valve provides fluid communication between the associated fluid separator unit and a reservoir. This vents pressure from the associated fluid separator unit allowing the piston thereof to move back to an unactuated position under the urging of an associated spring, thereby reducing pressure at the associated wheel brake.
The positions of the proportional control valves are controlled such that the pressure of fluid in the hydraulic circuit is controlled proportionally to the current of the energizing electrical signal. As a result, the exact position of a proportional control valve is proportional to the electrical control signal. Thus, the proportional control valves can be positioned at an infinite number of positions rather than just the three positions described above.
Traditional valve control encompasses two positions: open or closed. In a closed position, ideally, there is no flow through the valve. In the open position, there is flow through the valve in proportion to the degree the valve is opened. Voltage across a controlling solenoid dictates the amount of valve opening. There are also two traditional types of valves that are used in hydraulic systems: spool valves and poppet valves. A major difference between poppet valves (used herein) and spool valves is that there is significant amount of leakage associated with poppet valves. How to deal with poppet valve leakage is therefore important to the performance of an EHB system. A common practice is to minimize the effect of leakage through careful mechanical design and software algorithm. However, valve leakage may also be used to increase system resolution, reduce valve clicking noise if a valve is supposed to be open, while being eliminated to minimize flow consumption if a valve is supposed to be closed. This creates a system where the beneficial aspects of valve leakage can be taken advantage of while the negative aspects thereof can be suppressed.
The terms “bulk flow” and “leakage flow” are used to describe the primary types of flow through valves as used in this application. “Bulk flow”, as used herein, means the flow in a valve that occurs when the moving valve element (such as the valve poppet in poppet valves) is off of its seat. “Bulk flow” can also be described as flow through an open valve. “Leakage flow”, as used herein, means that flow that generally occurs during closed valve operation due to limitations in the manufacturing process and the design of the valve. Depending on conditions of operation as well as the type of valve and its manufactured characteristics, some flow can leak through it, even with the valve in a “closed” position. “Leakage flow” as used herein also means that flow which occurs when the moving element is not fully seated against its seat or is only intermittently in contact with the seat (e.g. “chattering” or “simmering”). Either leakage or bulk flow can be “laminar flow” or “turbulent flow”; however, it can be expected that bulk flow will be turbulent flow. Laminar flow is classically defined as a well ordered pattern of flow whereby fluid layers are assumed to slide over one another. Turbulent flow is irregular or unstable flow.
While the above-described system and other existing systems have effectively managed the operation of valves in an EHB system, there is a need for greater incremental control of the valves, as well as accounting for the various flow states through the valves. One limitation in controlling valves in a closed or near-closed position is that it is difficult to control the change in pressure applied to the brake. As a result, pressure to a brake would increase more than demanded by the vehicle user. There also is a need to account for the noise or clicking of valves when fluctuating between a near closed and closed position. The algorithm described below provides finer control of proportional valves in a braking system by controlling the voltage applied to operate the valve. Also described, as part of the invention, is the method of operating the valves to account for leakage and bulk flow as well as preventing flow while in a closed state.